aw_timer: rename driver from a10_timer

This reflects its expanded support for platforms other than the A10.

Functions specific to A10/A13 retain the a10 prefix, but the majority of
names in the driver are changed, e.g. a10_timer_softc becomes
aw_timer_softc.

Reviewed by: manu, ganbold
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48671